Selecting the correct inverter drive brings long-term benefits
ABB has published a booklet to help users select the most appropriate ac variable speed drive for their application. Curiously called the ABB micro drives : Drive dimensioning guide, the twelve page guide explains the impact of varying application and environmental considerations that affect a drive’s performance.
Selecting (dimensioning) the drive correctly involves a process of derating the continuous drive output current (motor current) or by selecting auxiliary devices such as chokes or RFI filters. Derating must be taken into account when using the drive in conditions as those shown in this guide.
Conditions that are often forgotten are the application cyclicity and switching frequency. Ambient temperature or other environmental conditions such as dust or humidity might also be unknown, especially when a drive is not installed by the end-user but the machine manufacturer.
It is very important to identify all derating factors in order to reach a reliable solution. Correctly dimensioning a drive saves you time and costs. When aiming for long-term reliability, these conditions might affect the rating of the drive. A drive which is correctly selected will work properly throughout its lifetime.
Conditions that require special consideration are outlined in the following table.
|Ambient temperature||Refers to the cabinet temperature in IP20 drives and the room temperature in wall-mounted drives.||Derating needed above 40 °C .|
|Switching frequency||The higher the switching frequency of the drive is, the more the power semiconductors heat up.||When using a higher switching frequency, a drive’s output current needs to be derated.|
|Altitude||Low air pressure decreases the cooling capability of air and air insulation.||Derating above 1000 m and PELV above 2000 m to be considered.|
|Duty cycle and application cyclicity||Primary target of micro drives are quadratic torque and simple constant torque applications, so attention needs to be paid when using them in cyclic applications.||When used in demanding constant torque applications, the drive should be oversized in order to maintain lifetime expectation.|
|Motor cabling||Long motor cabling requires special consideration because high frequency common mode current causes drives to overheat.||Drive derating, external output choke or EMC need to be considered.|
|Multi-motor systems||When several motors are powered by one drive, the common mode current increases and therefore current also increases (because of parallel connection of motor windings).||The same dimensioning criteria as for single motor drives doesn’t apply anymore.|
|Supply network quality||The drive usually has a limited amount of input reactance and therefore unstable power supply causes stress for the drive’s input bridge.||Input choke enhances the reliability of the drive.|
|Harsh environment||The drive is constantly or occasionally exposed to dust and/or humidity or even splashing or spraying water.||IP66/67 variant needs to be selected. Also, proper filtering and preventing hot air circulation inside the cabinet are useful methods.|
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